Over many years, the definition of inverters has become blurred, mostly around the term “hybrid inverter” For the lay person an “off grid inverter” may bring to mind a camping trip or small systems. But there are some big differences in how the inverter works and its function which confuses most.
Hybrid inverters which run homes are grid tied and grid parallel. For the most part they have a battery and provide mains power. The key part of this type of inverter is “grid parallel” which means:
Hybrid inverters work only with the grid – if the grid down so are you.
Hybrid inverters work in parallel and import and export power on the same line.
Hybrid inverters are NOT grid forming but work “together with and in parallel with the grid”.
Hybrid inverters may have a EPS – a offline inverter to make ac power in the event of a grid outage.
A Grid attached inverter is pretty much the same and nothing alike.
A Grid attached inverter plugs in. Much the same as a kettle or more so like a UPS that is used to power servers.
A Grid attached inverter does not feed power back to the grid, so there is no export of power, this is the big difference along with the fact that it does not work in parallel with the grid, therefore if the grid down you dont go off line as the inverter is grid forming.
Just so that you understand, grid forming is a off grid “thing” in that the inverter has to make AC power and control the AC power that it makes. This is different to grid parallel where the inverter has to sync with the grid and its voltage and frequency. Most grid tied inverters and hybrid inverters do not form a pure sine wave themselves but use the grid to do so, this is why they go off line when the grid goes off.
What is the benefit of a grid attached inverter?
Grid attached inverters cost less, are always on and work pretty much the same as other inverters. you do not have to fill out paperwork or get permission to install the inverter from the DNO as you would with parallel grid tied inverters.
The inverters operate in a number of ways, one being a UPS, this means that the battery is charged and only used when the grid goes off line, which makes this a UPS.
There is a Solar Battery Grid (SBU) which uses solar power firstly to power your load, then the battery and failing this the grid.
In both cases there is a pass through, the pass through is where the grid passes directly though the inverter into the load (your home) with the use of switches and relays that are build in. You do have to take care of how much power you can pass through the inverter as they can be limited.
The attachment to the grid is for two functions.
1 Pass through power to the load.
2 Charge the battery.
What are potential problems with using a grid attached inverter?
The first issue with using a grid attached inverter is the pass through. The limit is based on the transfer switch and the current that is allowed. For most homes, they can use 100amps of grid power, thats 23kw. Not all inverters are equal, therefore you can get 16 amp, 30amp and 65 amp. At 65amp some homes are limited to this amount of power which is around 15kw
A good example is the Victron Muliplus II which has a 32 amp pass though. which is around 7.4kw while only being a 2.4kw inverter, However this can operate as a parallel inverter, as well as a UPS and off grid inverter.
There is a general Lack of information in respect of grid charging and pass through, and this is due to problems with a wide range of inverters due to earth bonding on some electrical systems. Many of the off grid inverters will “pop a breaker” when the AC is connected due to the PVE natural bond at the inverter.
I should explain this in some more detail.
The problem arises when you try to connect an inverter that uses a different type of earthing system, called ‘TT’, to your home’s regular electrical setup (TN-C-S). Inverters with a TT system have the neutral and earth wires joined together inside the inverter.
Now, remember how in your home’s electrical system, the neutral and earth are already connected together before the electricity even enters your house? When you then plug in this TT inverter, it’s like creating a second connection between the neutral and earth wires within your home’s system.
Think of it like having two points on a water pipe that are already connected. If you then try to connect them again at a different point, it can cause problems with the flow and pressure.
In this electrical scenario, this ‘double connection’ between neutral and earth looks like a fault – specifically, a short circuit between neutral and earth – to your home’s protective devices, like circuit breakers and RCDs (Residual Current Devices). These devices are designed to trip and cut off the power when they detect such faults to prevent electric shock.
So, when you connect the inverter to your home’s distribution box, the RCD and breakers see this extra neutral-earth connection coming from the inverter as a problem. They interpret it as electricity leaking to earth, even though it’s part of the inverter’s normal TT operation. This is why they immediately trip, effectively preventing the inverter from supplying power to your home or power to your inverter from the grid.
To over come this problem inverters use relays to switch the earth neutral bond inside the inverter thus avoiding the problem, but Not all inverters do this. You can avoid this by removing the earth to the inverter and having an independent earth. However it is likely that you will have bonds else where which may cause an issue.
You can read more here on earthing arrangements:
TT earthing considerations
“Where above ground, special care must be taken to avoid simultaneous contact of conductive parts connected to different earthing systems as a potential difference may exist between both systems. In some cases, it may be necessary to convert the whole installation to a TT earthing system“
Can I detach the earth from the grid to the inverter and make my own earth system?
Yes, your system would be off grid and there is no need to have connected earths to the main grid, however care should be taken as the potential difference, which means voltage difference between other grid systems could cause a shock. Typically an inverter works at 230v where as grid power can be working around 245v. Therefore if there is a join between the two there is a 15v difference which you may not feel unless there is a fault with either system or a system near by. For most applications this should not be an issue, however it is a required check for the installation.
More reading:
Metal consumer units in an installation forming part of a TT system earthing arrangement | NICEIC – Professional Electrician
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